1. Field of the Invention
The present invention relates generally to systems for injecting chemicals into pipelines and, more specifically, to an improved system and method for adding odorant to natural gas or liquified petroleum gas flowing in a pipeline.
2. Description of the Prior Art
There are many instances in which it is desirable to inject chemical of various types into fluids (gas and liquids) flowing in pipelines. One such example is in the area of natural gas pipelines. In addition to such substances as corrosion inhibitors and alcohol to inhibit freezing, odorants are commonly injected into natural gas pipelines. Natural gas is odorless. Odorant is injected into natural gas in order to provide a warning smell for consumers. Commonly used odorants include tertiary butyl mercaptan (TBM). Such odorants are typically injected in relatively small volumes normally ranging from about 0.5 to 1.0 lbs/mmscf.
The odorants are typically provided in liquid form and are typically added to the gas at a location where distribution gas is taken from a main gas pipeline and provided to a distribution pipeline. In such circumstances, the gas pressure may be stepped down through a regulator from, for example, 600 psi or more, to a lower pressure in the range of 100 psi or less. The odorants can also be added to the main transmission pipeline in some situations.
As will be apparent from the above discussion, the odorants which are added to natural gas are extremely concentrated. Odorants such as TBM and other blends are mildly corrosive and are also very noxious. If the job of injecting odorant is not performed accurately, lives are sometimes endangered. It would be possible for a homeowner to have a gas leak with a leak not being realized until an explosion had resulted if the proper amount of odorant was not present.
Also, if a leak of odorant occurs at an injection site, people in the surrounding area will assume that a gas leak has occurred with areas being evacuated and commerce being interrupted. Contrarily, if such mistakes become common, people in the surrounding area will become desensitized to the smell of a potential gas leak and will fail to report legitimate leaks.
Two techniques are commonly used for providing odorization to natural gas in a main distribution pipeline. The first technique involves the injection of liquid odorant directly into the pipeline. A high pressure injection pump pumps the odorant from a liquid storage tank into a small pipe which empties directly into the main gas pipeline. Because odorant is extremely volatile, drops injected to the pipeline immediately disperse and spread throughout the gas in the pipeline. In this way, within a few seconds, the drops of liquid odorant are dispersed in gaseous form. U.S. Pat. No. 6,208,913, issued Mar. 27, 2001, to Marshall and Zeck, and U.S. Pat. No. 5,490,766, issued Feb. 13, 1996, to Zeck, both show state of the art fluid pumps for injecting odorant.
In the prior art systems, the flow of gas in the pipeline is typically metered so that liquid odorant can be injected periodically. For example, a few drops of odorant will suffice for a 1000 SCF flow of natural gas. When the gas flow meter indicates that 1000 SCF of natural gas have flowed through the pipe, the corresponding previously determined amount of liquid odorant is injected into the pipeline. As another 1000 SCF of gas flows past the injection site, another injection of odorant is performed. Even though the injection is performed on a periodic basis, the odorant diffusion within the gas provides for adequate levels of odorant throughout the pipeline, assuming the time between injections is not too great.
There are several disadvantages with this prior art technique. As mentioned above, the odorant liquid is extremely noxious. The injection pump must therefor be designed so that no odorant can leak. This requires a pump design which is relatively expensive and complex in order to meet the required operating conditions. In even such sophisticated systems, there is an unpleasant odor present when working on the pump which can make people think that there is a natural gas leak.
Another technique for odorizing a natural gas pipeline involves bypassing a small amount of natural gas at a slightly higher pressure than the pressure of the main distribution pipeline, through a tank containing liquid odorant. This bypass gas absorbs relatively high concentrations of odorant while it is in the tank. This heavily odorized bypass gas is then placed back into the main pipeline. The odorant, now volatalized, is placed back into the main pipeline and diffuses throughout the pipeline in much the same manner as described with respect to the liquid injection system. U.S. Pat. No. 6,142,162, issued Nov. 7, 2000, to Arnold, shows such a method for odorizing natural gas in a pipeline utilizing bypass piping in conjunction with a liquid odorant storage tank.
One disadvantage of the bypass system is the fact that the bypass gas picks up large and inconsistent amounts of odorant from the liquid in the tank and becomes completely saturated with odorant gas. As a result it is necessary to carefully monitor the small amounts of bypass gas which are used. Also, natural gas streams typically have contaminates such as compressor oils or condensates which can fall out into the odorant vessel in bypass systems. These contaminates create a layer that reduces the contact area between the liquid and the bypass stream. This necessarily degrades the absorption rate of the stream.
In U.S. Pat. No. 7,056,360, issued Jun. 6, 2006, entitled “Optical Odorization System”, by Zeck, there is shown an improved system for odorizing natural gas flowing through a pipeline by injecting odorant into the pipeline at a controlled rate. The system includes an odorant storage tank containing an odorant to be injected. A pressurized source of inert gas, such as nitrogen, communicates with the odorant storage tank for maintaining the tank at a desired positive pressure above the pressure of the natural gas pipeline. An injection conduit communicates the odorant storage tank with the pipeline. A photooptic metering means, located within the injection conduit, meters odorant to be injected into the pipeline.
The odorization system described in pending Ser. No. 10/287,392 was extremely simple as compared to existing positive displacement pump systems. The system offered improved ease of understanding, operation and maintenance. As compared to the prior art systems, the system described therein offered more accurate control, verification (drop counting) and communication (alarms, status, etc) than the prior art systems. The system offered generally smell free operation and maintenance.
Despite these advantages, the previously described optic system was necessarily limited in some respects by the nature of the optic system. For example, a window or view glass was required into the drop chamber which added to the weight and expense of the system. It was not possible to odorize liquified petroleum gas, since the system depended upon drops being metered into a gaseous medium. Similarly, since it was not possible to measure both drops and steady flow, the “turn down” ratio of the system was limited.
There continues to be a need for improvements in odorization systems of the above described types.